This is a continuation-in-part of our co-pending application Ser. No. 369,298, filed June 12, 1973, which application is abandoned upon the filing of this application.
It is generally known in the art to use as bleaching agents or brightening agents for high yield fiber pulps for papermaking, certain oxidizing or reducing agents. Among such agents which, when added to the fiber pulp, increase the brightness of the paper, are different kinds of peroxides, as well as various salts, for example, salts of sulfurous acid, sulfite and bisulfite, and hydrosulfites. These bleaching agents are relatively expensive, and it is generally known that the efficiency and the degree of their utilization is increased considerably if the bleaching process is carried out with pulps of higher concentration of solids, i.e. with less water in the aqueous pulp suspension. In order to attain this favorable effect without having to overdose the chemicals due to dispersion problems, it is essential that the chemicals which are added to the high-consistency pulp are rapidly and uniformly dispersed over each individual fiber contained in the pulp suspension. For this purpose, mixing apparatus such as rapidly rotating disc refiners have heretofore been used, into which refiners the chemicals are introduced either before or during the passage of the pulp suspension through refiners or mixing discs.
In addition to disc refiners, a number of various constructions of so-called mixers have been used, which comprise arms which rotate at suitable speeds to agitate the high consistency fiber pulp. Common for these mixing apparatuses, however, is the fact that the purely mechanical treatment causing the dispersion and distribution of the added chemicals also acts in a mechanical way on the fibers contained in the pulp suspension and thereby produces changes in the so-called ground state of the fibers which may be less desirable in some situations.
Furthermore, the power required for the mechanical admixing process is high, although, in spite thereof, the efficiency of the admixing action is relatively low. Therefore, it has been necessary when using earlier known apparatus to overdose the quantity of added chemicals in order thereby to compensate for the lack of instantaneous effect of the intermixing action.